A disk drive is a digital data storage device that stores information on concentric tracks on a storage disk. The storage disk is coated on one or both of its primary surfaces with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a constant rate. To read data from or write data to the disk, a magnetic transducer (or head) is positioned above (or below) a desired track of the disk while the disk is spinning.
Writing is performed by delivering a polarity-switching write current signal to the magnetic transducer while the transducer is adjacent to (above or below) the desired track. The write signal creates a variable magnetic field at a gap portion of the magnetic transducer that induces magnetically polarized transitions on the desired track. The magnetically polarized transitions are representative of the data being stored.
Reading is performed by sensing the magnetically polarized transitions on a track with the magnetic transducer. As the disk spins below (or above) the transducer, the magnetically polarized transitions on the track induce a varying magnetic field into the transducer. The transducer converts the varying magnetic field into a read signal that is delivered to a preamplifier and then to a read channel for appropriate processing. The read channel converts the read signal into a digital signal that is processed and then provided by a controller to a host computer system.
When data is to be written to or read from the disk, the transducer must be moved radially relative to the disk. In a seek mode, the transducer is moved radially inwardly or outwardly to arrange the transducer above a desired track. In an on-track mode, the transducer reads data from or writes data to the desired track. In the on-track mode, the transducer is moved radially inwardly and outwardly to ensure that the transducer is in a proper position relative to the desired track. The movement of the transducer in on-track mode is referred to as track following.
Modern hard disk drives employ a dual-actuator system for moving the transducer radially relative to the disk. A first stage of a dual-actuator system is optimized for moving the transducer relatively large distances. A second stage of a dual-actuator system is optimized for moving the transducer relatively small distances.
Upon initial manufacture and assembly into a hard disk drive, a storage disk contains no information. A low-level formatting step is performed at the factory during which enough data is written to the disk to allow for subsequent partitioning and high-level formatting by the computer operating system. In particular, during low-level formatting, a specialized system writes control structures to the storage disk that outline the positions of the tracks and sectors of the disk. The control structures include embedded servo data to control the head actuator as will be described in detail below.
After control structures defining the positions of the tracks and sectors have been written to the storage disk, certain predefined test patterns are written to the disk and then read back to determine the existence and location of any location specific anomalies on the storage disk. The term “location specific disk anomalies” as used herein refers to any flaw or defect in the storage disk itself or any external factor that repeatedly disrupts the hard disk drive system at a particular location within the previously defined system of tracks and sectors. This process is referred to as the flaw scan process.
One common example of a location specific disk anomaly includes defects in the magnetic storage media that alter the magnetic properties of the storage disk itself (media defects). For example, if the material at a particular location on the disk is not of the proper composition, the material may not be properly magnetized during the write process such that the data at that location may not be read during the read process.
Another example of a location specific anomaly is commonly referred to as a thermal asperity (TA). A thermal asperity is caused by some factor that causes the temperature of the magnetic transducer to increase. An increase in the temperature of the transducer interferes with the read signal generated thereby. Common causes of thermal asperities include perturbations in the surface of the disk and/or external contaminants on the disk surface that frictionally engage the head and cause brief spikes in head temperature.
During a conventional flaw scan process a single test pattern is written to the entire disk and then read back. A typical test pattern, referred to as the T4 pattern, is a square wave pattern which has 50% duty cycle and has a cycle of four times that of the read/write clock. The read signal generated by the magnetic transducer in response to such test pattern is a sinusoidal-like signal that is analyzed for disruptions indicative of media defects and/or spikes indicative of thermal asperities. The locations of any such location specific disk anomalies are stored, and any data sectors containing such anomalies are mapped out as bad sectors.
Although the conventional flaw scan process works well with Giant Magneto Resistive (GMR) heads, newer disk drive systems employ Tunneling Magneto Resistive (TuMR) heads. The response to TAs from TuMR heads has much smaller amplitude and shorter duration than that from GMR heads. Read channels that were designed to detect TAs from GMR heads are much less sensitive to TAs from TuMR heads. Missing a real TA in flaw scan could lead to serious consequences. For example, the integrity of user's data may be compromised and/or the read head may be damaged because of repeated contacts with the contaminants that cause the TA itself.
One way to enhance the sensitivity of the TA detector in the read channel is to apply a greater gain in the variable gain amplifier (VGA) in the analog front end (AFE). However, the increased gain would also cause the amplitude of signals free from TA to be large enough to saturate the dynamic signal range of the analog to digital converter (ADC). Consequently, the signals free from TA suffer distortion after ADC and may not be appropriate for defect detection.
One of the important differences between TAs and defects is that TAs can be observed from the read signal regardless of the data pattern written in the media. Therefore, a test pattern may be written that results in a read signal of very small amplitude when there is no TA. One example of such test pattern is the DC pattern, referred to as the T0 pattern. Such signals cannot be used to detect defects, but a greater VGA gain can be applied to enhance TA detection sensitivity.
Using the conventional flaw scan techniques, two read-passes would be necessary to capture all media defects, because the T0 pattern is insensitive to magnetic defects but effectively uncovers TAs, and the T4 pattern (or and other higher-frequency patterns) is less sensitive to TAs but pick out media defects well. For each track, one write/read pass would be conducted with one test pattern such as a T4 pattern and reduced VGA gain to enable accurate detection of media defects, and a second write/read pass would be conducted with another test pattern such as a T0 pattern and increased VGA gain to enable accurate detection of thermal asperities. Compared to the situation with a GMR head where one write/read pass for each track with the T4 pattern is sufficient to detect both media defects and TAs, this approach doubles the flaw scan process and therefore results in longer time and higher cost in the manufacturing process.
The need thus exists for flaw scan systems and methods that effectively detect both media defects and thermal asperities, while allowing the duration of the flaw scan process to be kept at acceptable levels.